To create the base tier of the cake, cut 3 slices of watermelon, 1 to 2 inches thick, crossways through the center of the melon. Trim the rind from the watermelon slices, and place the largest slice onto a cake plate. Alternate layering sliced strawberries, slices of cantaloupe, and slices of fresh pineapple; follow with the second-largest watermelon slice. Repeat once more to make a layered fruit stack that looks like a cake.
Thinly slice a few pieces of remaining melon and pineapple. With cookie cutters or a sharp knife, cut remaining fruit slices into decorative shapes. Decorate the cake tiers with the cut fruit shapes and remaining berries.
Refrigerate until serving time. To serve, slice the cake with a sharp knife, one wedge at a time, and set the wedge onto a plate.